The word is commonly pronounced in English as /fiænˈtʃɛtoʊ/ (soft "ch" as in "cherry") but the original word is pronounced in Italian as /fjaŋˈketto/ (hard "ch" as in "chaos").
It is common for terms borrowed from other languages to take on 'erroneous' pronunciations if the orthography is ambiguous (or completely different) in the loanee language, e.g.
When in Rome speak Roman.
They may say fi an Ket oh in Italy, but the rest of the world says fi an CHet oh.
Words change meanings and pronounciations over time and the majority usage eventually wins out.
I always have heard fi an Chet oh and that is how I say it.
According to Wikipedia:
In chess, the fianchetto (/ˌfiænˈtʃɛtoʊ/; Italian:
[fjaŋˈketto] "little flank") is a pattern of development ...
Hence English speakers pronounce the "ch" as in "chess" and Italian speakers as in "kettle".
Which suggests that there is no "true, and proper, pronunciation".
While I'm not sure exactly how "fēənˈketō" would be pronounced (I'm not a native English speaker), that's more correct as to the pronunciation of the "ch" (following the Italian pronunciation).
The pronunciation written in English would be something like fee-ahn-keh-toh, with the accent on the third syllable (keh).
I just found this website in which there ...